Presently, most marking of information on food items, such as fruit, is done by use of direct contact printing or labeling with stickers. Both techniques present their own difficulties. Good print quality using direct contact printing is difficult to achieve when printing on nonuniform, irregular surfaces, as are presented when one attempts to print on oranges, apples, grapefruit, and the like. Placing labels on such fruits or vegetables can be difficult for the same reasons, and further requires the consumer to remove the label before consuming a food item, such as an apple, or the like.
It is often useful to place variable information on food items. One example of such a need, generally in the case of fruits or vegetables, or citrus fruit in particular, is the desire by suppliers to mark the fruit with the "Price Look-Up" (PLU) Code. This PLU code is important at the retail level for entering the proper price, based on fruit size. A quick, easy manner of placing such information on food items is presently desired. Also, desirable for brand recognition, is the ability to make images on the food item, as by ink printing a corporate logo or the like.
Ink jet printing would offer the advantage of increased speed of marking food products and the opportunity to put variable information on food items, such as fruits and vegetables.
Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Ink jet printing systems are generally of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At this break-up point, the droplets are charged in accordance with digital data signals. These drops are then passed through an electrostatic field which adjusts the trajectory of each droplet. The droplets are either directed back to a gutter for recirculation or to a specific location on the substrate to create the desired character matrix.
In drop-on-demand systems, a droplet is expelled under pressure from a valve directly to a position on the substrate in accordance with the digital data signals. A droplet is not formed or expelled unless it is to be jetted to the substrate. Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream system.
A problem associated with the direct marking of food items, such as packaged fruit, however, is that the information on the fruit must remain readable under a variety of conditions and must be stable, even in the presence of moisture. Because packaged or boxed fruit is often subjected to changing temperature and humidity conditions during transport, the surface of the fruit can develop condensation or sweat. The printed information on the surface of the fruit must remain readable even under such temperature and humidity extremes and sudden changes therein. The appearance of the printed image must not bleed, disappear, fade, or transfer to other fruit placed in contact therewith, under any of the conditions and environmental changes that fruit may encounter. For example, FD&C Blue #1 (CI Number 42090) is a major colorant in making violet or blue citrus inks. However, its sensitivity to water makes a printed image formed therefrom on citrus "bleed" when condensation occurs without the proper added ingredients. This is true to varying degrees with all FD&C dyes, due to their water solubility.
To attempt to address this need for noncontact printing on fruits and vegetables, some have suggested the use of ink jet printing. The problem, however, has been that ink used in such a fashion must meet the aforementioned rigid requirements, but also must be capable of being printed by use of existing ink jet printing equipment. Further, the printed image must be of food grade quality, edible and compliant with all applicable regulatory requirements such as those set forth by the U.S. Food and Drug Administration in the Code of Federal Regulations (CFR). In PCT publication number WO92/14795 the use of certain ink jet formulations has been mentioned in which a necessary component of the ink is methyl ethyl ketone ("MEK"). However, the use of such a solvent for marking fruits or vegetables is not desirable because it is not deemed acceptable for such food contact and therefore does not comply with certain governmental regulations, such as the regulations in the CFR that relate to acceptable ingredients for making inks for marking fruits and vegetables (Part 73, Chapter 1, Volume 21 of the CFR).
The aforementioned PCT application also suggests the use of ink compositions that employ acetone in a mixture with water or an alkanol and/or an alkyl ester, for ink jet printing onto a foodstuff. A cellulosic material is used as a thickening and binding agent. The formulation may optionally contain shellac or a rosin ester or certain other film-forming resins. The PCT publication expressly states that problems related to precipitation of dyestuffs and spreading of printed messages were experienced when the alcohol content was beyond 50%. Although acetone may be a suitable alternative to make an ink, its high evaporation rate makes long-term printer operation a problem. This problem is caused by the requirement of accurately maintaining the solvent balance of the ink while running in the printer. The required make-up would need to be robust enough to keep the solvents in the ink properly in equilibrium. This is especially true if the ink were required to operate at different environmental conditions of temperature and humidity.
An ink using acetone and water as the carrier, as suggested in the PCT publication, that would operate at ambient conditions of temperature and humidity, is difficult to employ in practice. Such an ink is even more difficult to use if it includes an alcohol or combination of alcohols, because of the problems in designing a suitable make-up solvent. If alkyl esters are included or substituted for the alcohols in the ink (with acetone still present), the job becomes even more complicated. The elimination of acetone from the ink makes the creation of a make-up much simpler.
Accordingly, there remains a need for a high quality ink jet composition that can be used to print by ink jet printing onto the surfaces of fruits and vegetable items, that will fulfill the rigid requirements as discussed above and yet be suitable for such a food-contact use, and thus comply with the list of acceptable ingredients as provided by governmental authorities such as the U.S. FDA.
Such an ink must be capable of providing printed images with exceptionally good moisture resistance, while employing components that are suitable for food contact, such as FD&C water-soluble dyes.